VXI VT1538A user manual About Period and Frequency Measurements

About Period and

Frequency

Measurements

The E1538A actually measures signal period for both the period and frequency functions. If the measurement function is set to frequency rather than period, the SCP returns the reciprocal of the measured period. The resolution of each period measurement is based on the time processor chip’s timer period (238.4nS). To improve resolution on faster input signals, multiple signal periods can be measured and averaged. For period measurements there are two different modes that can be used to control the number of periods to average. For frequency measurements only the APERture mode is available.

1.The [SENSe:]PERiod:NPERiod mode explicitly sets the number of signal periods to measure and average. The time it takes the SCP to return a reading is dependent on the input signal period (for a given NPERiod setting), longer signal periods take longer to return a reading.

In NPERiod mode the actual measurement resolution (in seconds) is fixed while the relative resolution (as a percentage of the input signal period) is variable. That is, when NPERiods is set to provide an adequate resolution for short period signals, long period signals will have increased resolution.

2.The APERture mode sets a fixed duration that the SCP will use to measure multiple signal periods. The actual effective APERture

<time> will be:

INT	æ-------------------------------	<time>	-ö	´ signal_period .
	èsignal_periodø

The minimum aperture will be 1 signal period, and the maximum will be 255 signal periods.

In APERture mode, the effective resolution (in seconds) varies with the period of the input signal. That is, as the signal period is reduced, the number of measurements averaged increases, thereby improving the effective resolution. However, the relative resolution (as a percentage of the input signal period) is fairly constant with changes in signal period.

Generally, more measurements (greater NPERiod count or longer APERture time) means a more accurate frequency value. Of course more measurements means that the reading returned contains more latency (is “older” in relation to the signal's current frequency). To track fast changing frequency, you have to trade-off some accuracy with a shorter aperture time.

16 Agilent E1538A Enhanced Frequency/Totalize/PWM SCP